Electrical machines transform mechanical energy into electrical energy (generator) and vice versa (motor). Electrical machines usually comprise a rotor and a stator. The rotor rotates within a bore that is provided in the stator. When used in generators, such a rotor may have a length from 1 meter up to 6 meters or even more. Also the diameter may vary from 250 mm up to 1200 mm or even more. Due to high rotational speeds and weight of the rotor, careful balancing is compulsory.
A lack of balance will lead to vibration. Vibrations are detrimental in view of lifetime of the mechanical components such as bearings etc. Furthermore it is not possible to pass a critical rotational speed with a rotor that is not balanced. If the balancing state of the rotor does not reach a certain degree, it is possible that the electrical machine will be damaged. Especially if the electrical machine will be operated over longer time period.
Balancing of a new rotor during construction of the same is not an issue, because this step is usually done in a high speed balancing pit. After planned or unplanned maintenance work during operation however, the step of balancing the rotor is a problem. Rewinding work is one maintenance operation that serves as an example. During such a procedure the rotor is removed from the bore of the stator. It is then shipped or transported to a workshop of the manufacturer. In the workshop of the manufacturer the rewinding work, replacing step or any other maintenance work will be carried out. However it has to be noted that it is possible to carry out such a replacement and many other maintenance steps in the power plant itself, meaning on site. After the replacing step is finished, the rotor is often out of balance and the step of balancing the rotor becomes necessary. This step is nowadays conducted on a balancing pit located at the workshop of the manufacturer. Usually a power plant, in which the rotor is used, does not have an own balancing pit and therefore transporting the rotor from the power plant to the workshop of the manufacturer becomes compulsory for the balancing step. The step of transporting the rotor to the workshop of its manufacturer is usually time consuming, costly and leads to an even longer interruption of operation.